Consider
  import M( C( a,b,c ) )
where class C is defined as
  module M where
     class C x where
        a :: blah
        c :: blah

Tnen (Trac #15413) we'd like to get an error message only about
failing to import C( b ), not C( a,b,c ).

This was fairly easy (and local) to do.

Turned out that the existing tests mod81 and mod91 are adequate
tests for the feature.